This invention relates to electrical plasma jet torches and more particularly concerns a torch having an improved nozzle electrode.
A typical electrical plasma jet torch projects a stream of plasma of exceedingly high temperature and velocity that results from an electric arc that is maintained between a back electrode, commonly a cathode, and a front electrode, commonly an anode, which itself is formed with a bore or passage that provides a nozzle for the torch. Where the high velocity plasma stream is to be employed for spray coating, a coating powder is injected into the plasma stream within the nozzle passage and entrained in the projected stream for impingement upon a substrate to be coated. An exemplary high power torch of this type is described in detail in U.S. Pat. No. 3,823,302 to Muehlberger, which is assigned to the assignee of the present application.
Although the anode or nozzle electrode of such a torch is a complex high precision part, and therefor expensive, it has a relatively short life and must be frequently replaced. Life of the nozzle electrode is shorter as velocities and temperatures of the plasma stream increase and as harder and more abrasive powders are employed.
Two factors are most significant in limiting the life of the nozzle electrode, arc erosion and powder erosion. Arc erosion occurs at the point of impingement of the electric arc upon the nozzle electrode. It has been observed that arc erosion will limit the life of an anode in a relatively low velocity gun, Mach I and below, to approximately 10 hours, and to considerably less in high velocity, supersonic torches. Its detrimental effects are closely related to temperature of the electrode. Thus, arc erosion is handled by attempting to cool the nozzle electrode. Often a coolant liquid, such as water, is caused to flow in a jacket around the electrode. Cooling may not be adequate in such an arrangement, in spite of the high heat conductivity of copper electrode material, because of the tremendously high temperatures of the plasma stream within the electrode bore. A typical jacket arrangement for flow of coolant around the anode is shown in U.S. Pat. No. 3,242,305 to Kane et al.
In some cases, the anode itself is formed with a chamber that receives coolant flowing through the chamber to and from the anode. Typical of such an arrangement are the anodes shown in U.S. Pat. No. 3,390,292 to Perugini and in the above-mentioned U.S. Pat. No. 3,823,302. Such arrangements require the drilling of multiple, precisely spaced radial holes in the anode for input or output of water. Even so they do not provide adequate cooling at the front of the anode.
U.S. Pat. No. 3,740,522 to Muehlberger illustrates an arrangement of coolant passages, alternate ones of which flow coolant forwardly and the remaining ones of which flow coolant rearwardly. In the device of this patent a number of radially directed coolant input apertures must be precisely located to communicate with selected axially directed passages, and a groove communicating with the forward ends of all of the bores is closed by means of a soldered closure plug. It is found that the drilling and positioning of radially directed holes is time consuming and expensive. It is difficult to control and inspect the soldering of the closure plug so as to ensure a leak free seal. Improper soldering of the closure plug shows up during use, after the torch has been delivered to the customer, resulting in costly and otherwise undesirable repairs and replacements.
A still further limitation of use of the cooling arrangement of U.S. Pat. No. 3,740,522 derives from the fact that it is frequently desirable to employ a number of powder injection bores extending radially through the anode. With the cooling passage arrangement shown in U.S. Pat. No. 3,740,522 it is possible to employ but a single powder injection bore unless a number of the cooling passages are eliminated, thereby eliminating significant amounts of cooling.
Because the anode wears out and must be replaced with relative frequency, it is significant to keep the cost as low as possible and thus a single basic anode configuration is highly desirable, allowing the one anode "blank" to be completely formed in a common configuration except for powder holes and plasma passage shape. It will be understood that the shape of the plasma passage and the location orientation and number of powder holes may vary from one plasma torch to another depending upon power, velocity and powders to be employed. Thus, a torch having a plasma velocity of Mach I or less may have one nozzle passage and powder bore configuration whereas a torch providing a plasma stream of Mach II or Mach III may have a different nozzle configuration and preferably has other powder bore requirements. Therefore a cooling arrangement such as that of U.S. Pat. No. 3,740,522 prevents the use of a common blank because it permits only one powder bore.
The second major limitation on nozzle electrode life is powder erosion. This occurs with all powders, but is a greater problem with highly abrasive powders such as tungsten carbide for example. It is more pronounced at higher plasma velocities which require higher velocities of powder injection. Powder erosion is particularly manifested by erosion of the powder bore at the point where the powder bore meets the nozzle passage. As this inner end of the powder bore erodes, velocity, spray pattern and even direction of the entering powder change, resulting in undesirable characteristics of the spray coating. Powder erosion has been observed to occur at the inner end of the powder bore and within the nozzle passage itself at areas thereof downstream of the powder bore. To counter effects of powder erosion, a tungsten liner has been formed, inserted into the electrode passage and soldered therein. Holes are drilled substantially radially through the liner in registration with the powder bore that extends through the anode and thus, the relatively soft electrode material (generally copper) is replaced at the point of erosion, namely the inner end of the powder bore and the surface of the nozzle passage. It is replaced by a material that has significantly greater resistance to erosion. However, such a tungsten liner is difficult and expensive to install and itself adds to the very problem that it was designed to counter, namely, the high cost of the nozzle electrodes over the life of the torch. In supersonic spray coating of certain powders, such as tungsten carbide, in the absence of an erosion resistant liner in the nozzle passage, significant powder corrosion has been found to occur within as little as 30 minutes of torch operation. A tungsten liner is a costly way of extending anode life as limited by powder erosion. Another adverse aspect of the use of an erosion resistant liner within the nozzle passage resides in the fact that this liner, like the coolant passage arrangement of U.S. Pat. No. 3,740,522, prevents standardization of the anode design and use of a common anode blank. This is so because the liner must be positioned within the anode at an early stage of the anode manufacture and thereafter change of powder bore position, orientation or number is no longer available. Thus, it will be seen that the powder erosion problem has been handled in the past by controlling the surface of the nozzle passage, but at greatly increased cost of manufacture.
The Patent to Unger et al, U.S. Pat. No. 3,313,908 discloses a front electrode formed as an insert and held in place by a set screw. In addition to the problems of the tungsten liner, dimensional stability, physical positioning and sealing with the arrangement shown in this patent are all significant problems. Further, cooling must be provided entirely by housing passages and no coolant chambers or passages are provided within the insert itself.
Accordingly, it is an object of the present invention to provide a plasma torch and nozzle electrode therefor that eliminate or significantly minimize the above-mentioned problems.